Most modern spacecraft or aircraft rely upon chemical propulsion. Chemical propulsion typically requires fuel and an oxidizer to be burnt to produce both energy and a reaction mass. Various engine designs have been utilized in order to convert the energy and reaction mass into thrust which propels the spacecraft or aircraft. However, the performance of these engine designs is limited by the energy of the chemical reactions and by the molecular mass of the reaction products, such as H2O and CO2.
As a result of the limitations upon the performance of these various engine designs, alternative approaches have been developed. Some alternative approaches have utilized an external energy source, such as a beam of electromagnetic energy. By relying upon an external energy source, these alternative approaches have eliminated the need for combustion such that the propellant may be chosen to have a relatively low molecular mass, therefor creating a relatively high exhaust speed for a given temperature. Since the materials from which an engine is constructed are limited by the peak temperature at which the engine operates, the reliance upon a beam of electromagnetic energy as a source of external energy allows higher exhaust speeds and, therefore, a higher specific impulse than engines that rely upon chemical propulsion.
One approach that utilizes an external energy source is a heat exchanger laser thruster. A heat exchanger laser thruster includes a set of tubes arranged on a flat plate upon which the beam of electromagnetic energy, such as a laser beam, impinges. Each tube carries a fraction of the propellant that flows from a tank into a nozzle of the engine. The outer surfaces of the tubes absorb the electromagnetic energy as heat. The heat flows through the wall of the tubes and into the propellant. The tubes may be formed of an opaque material, such as metal, that carries a relatively high heat flux from the outer surfaces at which the tubes absorb the electromagnetic energy to the inner surfaces that are in contact with the propellant.
As such, the heat exchanger laser thruster operates as a second surface heat exchanger. A second surface heat exchanger may impose conflicting requirements with respect to the material from which the tubes are constructed. In this regard, the material from which the tubes are constructed is heated during operation to be as hot as possible so that it can, in turn, heat the propellant to be as hot as possible. Additionally, the material from which the tubes are formed is desirably thin so as to reduce the temperature difference between the surface of the tubes upon which the beam of electromagnetic energy impinges and the propellant. These requirements for both thinness and operability at high temperatures may be in conflict with the requirement to contain the propellant under high pressure. In this regard, the propellant is required to be maintained under high pressure in order to achieve a desired thrust-to-weight ratio for the engine. Additionally, the requirement for the material from which the tubes are constructed to have a high strength at high temperatures may conflict with a need for the material from which the tubes are constructed to conduct a relatively large heat flux. For example, titanium may have relatively good strength at a high temperature, but does not conduct heat as well as aluminum.
In another example, a spacecraft or aircraft, such as a rocket or a jet engine, may include a windowed heat exchanger. In this regard, a beam of electromagnetic energy, such as a laser beam, may pass through a window and enter the engine. The window may be a single large plate of transparent material attached to an opaque structure. Once the electromagnetic energy has passed through the window, the electromagnetic energy may be either directly absorbed by the propellant or may impinge upon a surface that, in turn, is in direct contact with the propellant. The window of a windowed heat exchanger therefore both admits the electromagnetic energy and confines the propellant.
As a result of the containment of the working fluid by the single large plate of transparent material that serves as the window in a windowed heat exchanger, the thickness of the window increases with its width for a given pressure. Additionally, transparent materials that may otherwise be suitable for the windowed heat exchanger are rarely as stiff as standard structural materials such that the windowed heat exchanger having a relatively wide window may require a thick and, therefore, quite heavy, plate of transparent material.
Therefore, there is a need for improved optical heat exchangers that permit more payload to be delivered at a lower cost. In this regard, there is a need for an improved optical heat exchanger to allow the thrust-to-weight ratio of the engine of a vehicle to be enhanced in order to address at least some of the foregoing deficiencies.